Diversity and reproduction of near-shore vs offshore wood-boring bivalves (Pholadidae: Xylophagainae) of the deep eastern Pacific ocean, with three new species

The ability of obligate wood-boring bivalves of the Xylophagainae to colonize wood seemingly wherever it lays on the seafloor remains enigmatic. The continuous, if patchy, deposition of vegetation in near-shore deep-water areas is hypothesized to allow woodborers to develop opportunistic species that exploit offshore wood falls and to produce larvae that colonize isolated wood-falls on the distant seafloor. Examination of specimens and literature accounts from near-shore (within 1.5 degrees longitude of the continent) and offshore (more than 2.3 degrees longitude from the continent) areas tests the hypothesis that the same species occur in both areas. The hypothesized role of near-shore populations as sources of offshore colonists is refuted; the 18 species of Xylophagainae documented in the Northeast Pacific Ocean appear to form two nearly distinct groups based on their proximity to the continent. Of 11 near-shore species of Xylophagainae recorded off western North America, including three (Xylophaga siebenalleri n. sp.; X. pacifica n. sp., Xylopholas scrippsorum n. sp.) described here, only one is also known from offshore sites. Four near-shore species are documented to range from the San Diego Trough to Oregon, including three that apparently brood young. Brooded young are considered to restrict the offshore dispersal of species, but not to limit their movement along the continental margin.